Soda ash or sodium carbonate is a valuable mineral source. Soda ash is one of the largest volume alkali commodities made in the United States. Soda ash finds major use in the glass-making industry and for the production of baking soda, detergents and paper products.
The processes for the production of soda ash employ crystallization steps that concentrate impurities in the mother liquors. Purge streams are required in these processes to remove impurities. The purge or waste streams include soda ash as well as impurities, such as sodium bicarbonate, sodium chloride, and sodium sulfate. Typically, in the manufacture of soda ash, a system of storage ponds has been used to accommodate disposal of the effluent streams including purge streams, mine water, and other sources of waste waters inherent to the process. The effluent stream is transported year-round to a pond. During the summer, water evaporates from the pond and sodium carbonate decahydrate precipitates into a solid mass with impurities, including sodium bicarbonate and sodium sulfate. In the winter, crystals form by cooling crystallization, and are deposited at the bottom of the pond over the previously deposited summer precipitation. The evaporation process also concentrates the amount of sodium chloride and other impurities in the pond solution. The liquid in the continuously filled ponds typically has NaCl levels of greater than 11% in the summer and 14% in the winter.
The sodium carbonate decahydrate crystals (along with substantial impurities) deposited on the pond bottom reduce the total pond volume. Although these sodium carbonate decahydrate deposits may be removed from the pond, they are hard, dense, and difficult to remove. A substantial amount of sodium carbonate decahydrate resides in these ponds. If the mass is not removed, the solids eventually fill the available waste pond volume until an increase in pond volume must occur, either by raising existing dikes, expansion of existing waste ponds, or the construction of new ponds.
Previous patents have attempted to address the difficulties in recovering soda ash from impure solutions. U.S. Pat. No. 1,853,275 indicates that it is known that sodium carbonate decahydrate may be recovered from a brine solution by crystallizing the solution in a pond over the winter months. U.S. Pat. No. 4,564,508 teaches the use of a non-convective solar pond to crystallize sodium carbonate decahydrate from an impure sodium carbonate solution. The sodium carbonate decahydrate crystals travel to the bottom layer of the pond where they are dehydrated to sodium carbonate monohydrate. U.S. Patent Publication 2004/0197258 indicates that deposited sodium carbonate decahydrate present in evaporation ponds may be recovered by solution mining the deposits.
Thus, it would be desirable to find a way to process the effluent to allow recovery of sodium carbonate decahydrate of improved purity. Additionally, it would be desirable to have sodium carbonate decahydrate that can be more easily removed from the pond to recover the valuable soda ash.